The present invention relates to an aluminium-magnesium alloy in the form of plates and extrusions, which is particularly suitable to be used in the construction of large welded structures such as storage containers and vessels for marine and land transportation. For example, the plates of this invention can be used in the construction of marine transportation vessels such as catamarans of monohull type, fast ferries, high speed light craft, and jet rings for the propulsion of such vessels. The alloy plates of the present invention can also be used in numerous other applications such as structural materials for LNG tanks, silos, tanker lorries and as tooling and moulding plates. Plates may have a thickness in the range of a few mm, e.g. 5 mm, up to 200 mm. Extrusions of the alloy of this invention can be used for example as stiffeners and in superstructures of marine vessels such as fast ferries.
Alxe2x80x94Mg alloys with Mg levels  greater than 3% are extensively used in large welded constructions such as storage containers and vessels for land and marine transportation. A standard alloy of this type is the AA5083 alloy having the nominal composition, in wt %:
In particular, AA5083 alloy plates in the soft and work-hardened tempers are used in the construction of marine vessels such as ships, catamarans and high speed craft. Plates of the AA5083 alloy in the soft temper are used in the construction of tanker lorries, dump trucks, etc. The main reason for the versatility of the AA5083 alloy is that it provides good combinations of high strength (both at ambient and cryogenic temperatures), light weight, corrosion resistance, bendability, formability and weldability. The strength of the AA5083 alloy can be increased without significant loss in ductility by increasing the Mg % in the alloy. However, increasing the %Mg in Alxe2x80x94Mg alloys is accompanied by a drastic reduction in exfoliation and stress corrosion resistances. Recently, a new alloy AA5383 has been introduced with improved properties over AA5083 in both work-hardened and soft tempers. In this case, the improvement has been achieved primarily by optimising the existing composition of AA5083 alloy.
Some other disclosures of Alxe2x80x94Mg alloys found in the prior art literature will be mentioned below.
GB-A-1458181 proposes an alloy of strength increased relative to JISH 5083, containing a larger amount of Zn. The composition is, in wt %:
In the examples, ignoring reference examples, the Mn contents range from 0.19 to 0.44, and Zr is not employed. This alloy is described as cold fabricatable, and also as suitable for extrusion.
U.S. Pat. No. 2,985,530 describes an alloy for fabricating and welding having a much higher Zn level than AA5083. The Zn is added to effect natural age hardening of the alloy, following welding. The composition for plate is, in wt %:
In xe2x80x9cThe Metallurgy of Light Alloysxe2x80x9d, Institute of Metallurgy, Ser. 3 (London) 1983, by Hector S. Campbell, pages 82-100, there are described the effects of adding 1% of Zn to aluminium alloys containing 3.5-6% Mg and either 0.25 or 0.8% Mn. The Zn is said to improve tensile strength and to improve stress corrosion resistance in ageing over 10 days at 100xc2x0 C. but not in ageing over 10 months at 125xc2x0 C.
DE-A-2716799 proposes an aluminium alloy to be used instead of steel sheet in automobile parts, having the composition, in wt %:
More than 0.4% Mn is said to reduce ductility.
One object of the present invention is to provide an Alxe2x80x94Mg alloy plate or extrusion with substantially improved strength in both soft and work-hardened tempers as compared to those of the standard AA5083 alloy. It is also an object to provide alloy plates and extrusions which can offer ductility, bendability, pitting, stress and exfoliation corrosion resistances at least equivalent to those of AA5083.
According to the invention there is provided an aluminium-magnesium alloy in the form of a plate or an extrusion, having the following composition in weight percent:
By the invention we can provide alloy plate or extrusion having higher strength than AA5083, and particularly the welded joints of the present alloy can have higher strength than the standard AA5083 welds. Alloys of present invention have also been found with improved long term stress and exfoliation corrosion resistances at temperatures above 80xc2x0 C., which is the maximum temperature of use for the AA5083 alloy.
The invention also consists in a welded structure having at least one welded plate or extrusion of the alloy set out above. Preferably the proof strength of the weld is at least 140 MPa.
It is believed that the improved properties available with the invention, particularly higher strength levels in both work-hardened and soft tempers, result from increasing the levels of Mg and Zn, and adding Zr.
The present inventors consider that poor exfoliation and stress corrosion resistances in AA5083 may be attributed to the increased extent of precipitation of anodic Mg-containing intermetallics on the grain boundaries. The stress and exfoliation corrosion resistances at higher Mg levels can be maintained by precipitating preferably Zn-containing intermetallics and relatively less Mg-containing intermetallics on the grain boundaries. The precipitation of Zn-containing intermetallics on the grain boundaries effectively reduces the volume fraction of highly anodic, binary AlMg intermetallics precipitated at the grain boundaries and thereby provides significant improvement in stress and exfoliation corrosion resistances in the alloys of the present invention at the higher Mg levels employed.
The alloy plates of the invention can be manufactured by preheating, hot rolling, cold rolling with or without inter-annealing and final annealing of an Alxe2x80x94Mg alloy slab of the selected composition. The conditions are preferably that the temperature for preheat in the range 400-530xc2x0 C. and the time for homogenisation not more than 24 h. The hot rolling preferably begins at 500xc2x0 C. Preferably there is 20-60% cold rolling of the hot rolled plate with or without interannealing after 20% reduction. The final and intermediate annealing is preferably at temperatures in the range 200-530xc2x0 C. with a heat-up period of 1-10 h, and soak period at the annealing temperature in the range 10 min to 10 h. The annealing may be carried out after the hot rolling step and the final plate may be stretched by a maximum of 6%.
Details of extrusion processes are given below.
The reasons for the limitations of the alloying elements and the processing conditions of the aluminium alloy according to the present invention are described below.
All composition percentages are by weight.
Mg: Mg is the primary strengthening element in the alloy. Mg levels below 5.0% do not provide the required weld strength and when the addition exceeds 6.0%, severe cracking occurs during hot rolling. The preferred level of Mg is 5.0-5.6%, more preferably 5.2-5.6%, as a compromise between ease of fabrication and strength.
Mn: Mn is an essential additive element. In combination with Mg, Mn provides the strength in both the plate and the welded joints of the alloy. Mn levels below 0.6% cannot provide sufficient strength to the welded joints of the alloy. Above 1.2% the hot rolling becomes increasingly difficult. The preferred minimum for Mn is 0.7% for strength and the preferred range for Mn is 0.7-0.9% which represents a compromise between strength and ease of fabrication.
Zn: Zn is an important additive for corrosion resistance of the alloy. Zn also contributes to some extent to the strength of the alloy in the work-hardened tempers. Below 0.4%, the Zn addition does not provide the intergranular corrosion resistance equivalent to that of AA5083. At Zn levels above 1.5%, casting and subsequent hot rolling becomes difficult especially at industrial scale. For this reason the preferred maximum level of Zn is 1.4%. Because Zn above 0.9% may lead to corrosion in a heat-affected zone of the weld, it is preferred to use not more than 0.9% Zn.
Zr: Zr is important for achieving strength improvements in the work-hardened tempers of the alloy. Zr is also important for resistance against cracking during welding of the plates of the alloy. Zr levels above 0.25% tend to result in very coarse needle-shaped primary particles which decreases ease of fabrication of the alloy and bendability of the alloy plates, and therefore the Zr level must be not more than 0.25%. The minimum level of Zr is 0.05% and to provide sufficient strength in the work-hardened tempers a preferred Zr range of 0.10-0.20% is employed.
Ti: Ti is important as a grain refiner during solidification of both ingots and welded joints produced using the alloy of the invention. However, Ti in combination with Zr forms undesirable coarse primaries. To avoid this, Ti levels must be not more than 0.2% and the preferred range for Ti is not more than 0.1%. A suitable minimum level for Ti is 0.03%
Fe: Fe forms Alxe2x80x94Fexe2x80x94Mn compounds during casting, thereby limiting the beneficial effects due to Mn. Fe levels above 0.5% causes formation of coarse primary particles which decrease the fatigue life of the welded joints of the alloy of the invention. The preferred range for Fe is 0.15-0.30%, more preferably 0.20-0.30%.
Si: Si forms Mg2Si which is practically insoluble in Alxe2x80x94Mg alloys containing Mg greater than 4.5%. Therefore Si limits the beneficial effects of Mg. Si also combines with Fe to form coarse Alxe2x80x94Fexe2x80x94Si phase particles which can affect the fatigue life of the welded joints of the alloy. To avoid the loss in primary strengthening element Mg, the Si level must be not more than 0.5%. The preferred range for Si is 0.07-0.20%, more preferably 0.10-0.20%.
Cr: Cr improves the corrosion resistance of the alloy. However, Cr limits the solubility of Mn and Zr. Therefore, to avoid formation of coarse primaries, the Cr level must be not more than 0.3%. A preferred range for Cr is 0-0.15%.
Cu: Cu should be not more than 0.4%. Cu levels above 0.4% gives rise to unacceptable deterioration in pitting corrosion resistance of the alloy plates of the invention. The preferred level for Cu is not more than 0.15%, more preferably not more than 0.1%.
Ag: Ag may optionally be included in the alloy up to a maximum of 0.4%, preferably at least 0.05%, to improve further the stress corrosion resistance.
The balance is Al and inevitable impurities. Typically each impurity element is present at 0.05% maximum and the total of impurities is 0.15% maximum.
Methods of making the products of the invention will now be described.
The preheating prior to hot rolling is usually carried out at a temperature in the range 400-530xc2x0 C. in single or in multiple steps. In either case, preheating decreases the segregation of alloying elements in the material as cast. In multiple steps, Zr, Cr and Mn can be intentionally precipitated to control the microstructure of the hot mill exit material. If the treatment is carried out below 400xc2x0 C., the resultant homogenisation effect is inadequate. Furthermore, due to substantial increase in deformation resistance of the slab, industrial hot rolling is difficult for temperatures below 400xc2x0 C. If the temperature is above 530xc2x0 C., eutectic melting might occur resulting in undesirable pore formation. The preferred time of the above preheat treatment is between 1 and 24 hours. The hot rolling begins preferably at about 500xc2x0 C. With increase in the Mg % within the composition range of the invention, the initial pass schedule becomes more critical.
A 20-60% cold rolling reduction is preferably applied to hot rolled plate prior to final annealing. A reduction of at least 20% is preferred so that the precipitation of anodic Mg-containing intermetallics occurs uniformly during final annealing treatment. Cold rolling reductions in excess of 60% without any intermediate annealing treatment may cause cracking during rolling. In case of interannealing, the treatment is preferably carried out after a cold reduction of at least 20% to distribute the Mg- and/or Zn-containing intermetallics uniformly in the interannealed material. Final annealing can be carried out in cycles of single or multiple steps in one or more of heat-up, hold and cooling down from the annealing temperature. The heat-up period is typically between 10 min and 10 h. The annealing temperature is in the range 200-550xc2x0 C. depending upon the temper. The preferred range is in between 225-275xc2x0 C. to produce work-hardened tempers e.g. H321, and 350-480xc2x0 C. for the soft tempers e.g. O/H111, H116 etc. The soak period at the annealing temperature is preferably between 15 min to 10 h. The cooling rate following annealing soak is preferably in the range 10-100xc2x0 C./h. The conditions of the intermediate annealing are similar to those of the final annealing.
In the manufacture of extrusions, the homogenisation step is usually done at a temperature in the range 300-500xc2x0 C. for a period of 1-15 h. From the soak temperature, the billets are cooled to room temperature. The homogenisation step is carried out mainly to dissolve the Mg-containing eutectics present from casting.
The preheating prior to extrusion is usually done at a temperature in the range 400-530xc2x0 C. in a gas furnace for 1-24 hours or an induction furnace for 1-10 minutes. Excessively high temperature such as 530xc2x0 C. is normally avoided. Extrusion can be done on an extrusion press with a one- or a multi-hole die depending on the available pressure and billet sizes. A large variation in extrusion ratio 10-100 can be applied with extrusion speeds typically in the range 1-10 m/min.
After extrusion, the extruded section can be water or air quenched. Annealing can be carried out in batch annealing furnace by heating the extruded section to a temperature in the range 200-300xc2x0 C.